1. Field of the Invention
The present invention relates to a projection-type color image display apparatus. More particularly, the present invention relates to a single-plate projection-type color image display apparatus for producing a color display with a single LCD ("liquid crystal display") device without using a color filter.
2. Description of the Related Art
A projection type color image display apparatus incorporating a conventional LCD device (hereinafter, referred to as a "projection type color LCD apparatus") will be described. A projection type color LCD apparatus is expected to be further developed in the industry, because it can provide various advantages over a projection type CRT (cathode ray tube) display apparatus, e.g., it has a wide color reproduction range; it is small in size and light in weight, and thus highly portable; and it is not influenced by geomagnetism, and thus does not require a convergence adjustment. However, an LCD device used in a projection type color LCD apparatus does not normally emit light, requiring a separate light source be provided.
Display systems for such a projection type color image display apparatus include a three-plate system where three LCD devices are used for the primary colors, R (red), G (green) and B (blue) and a single-plate system where only one LCD device is used. A projection type color image display apparatus of the three-plate system includes an optical system for separating white light into R, G and B beams and three LCD devices for respectively controlling the R, G and B beams so as to form R, G and B images. The R, G and B images are optically superimposed on one another so as to produce a full-color display. In the three-plate system, the light emitted from the white light source can be efficiently used, and a color with high purity can be displayed. However, the system requires the color separating system and the color synthesizing system as described above, and the overall optical system becomes complicated, requiring a large number of components to be provided. Thus, the system is generally disadvantageous over the single-plate system in terms of the cost and the size of the apparatus.
On the other hand, a projection type color LCD apparatus of the single-plate system uses only one LCD device. In the single-plate system, the LCD device including an RGB color filter of a mosaic or stripe arrangement is projected by a projection optical system. For example, such a projection type color LCD apparatus is disclosed in Japanese Laid-Open Publication No. 59-230383. The single-plate system is suitable for a low-cost and small-size projection system as it requires only one LCD device, and the optical system is simpler than that of the three-plate system.
In the single-plate system, however, light is absorbed or reflected by the color filter, whereby only about 1/3 of the incident light can be used. Accordingly, the brightness obtained by the single-plate system with a color filter is about 1/3 of that obtained by the three-plate system using a light source having the same brightness as that used in the single-plate system.
One possible solution to the reduced brightness is to increase the brightness of the light source used. However, an increase in the light source brightness is associated with an increase in the power consumption, which is undesirable, particularly when the apparatus is used at home. When a color filter of an absorption type is used, light absorbed by the color filter is converted to heat. Therefore, the increase in the light source brightness not only increases the temperature of the LCD device, but also accelerates the discoloring of the color filter. Thus, to enhance the utility value of a projection type color image display apparatus, it is important to more effectively use the given light without undesirably increasing the brightness of the light source.
In order to solve the above-described problem associated with the single-plate projection type color image display apparatus, Japanese Laid-Open Publication No. 4-60538, for example, discloses a projection type color image display apparatus in which a plurality of dichroic mirrors are arranged in a fan-arrangement so as to improve the light efficiency.
The conventional projection type color image display apparatus improves the light efficiency by providing a plurality of dichroic mirrors 104R, 104G and 104B in a fan-shaped pattern for separating the white light from a white light source 101 into R, G and B beams, as illustrated in FIG. 38. As used herein, "R, G and B" refer to red, green and blue, respectively, and "R, G and B beams" refer to a red, green and blue light beams, respectively.
In the conventional apparatus, the light beams separated by the dichroic mirrors 104R, 104G and 104B are incident upon the microlens array 105 at respectively different angles. The microlens array 105 is provided on a side of a LCD device 107 closer to the white light source 101. After passing through the microlens array 105, the color beams are distributed, depending on their incident angles, to different liquid crystal regions (pixel regions) of the LCD device 107, which are driven by different signal electrodes to which different color signals are independently applied. The distributed light beams are projected while being enlarged onto a screen 110 via a field lens 108 and a projection lens 109, which are provided on a light output side of the LCD device 107. The conventional projection type color image display apparatus does not use an absorption-type color filter, and thus achieves an enhanced light efficiency, thereby displaying bright images.
Referring to FIG. 39, the LCD device 107 used in the conventional projection type color LCD apparatus includes two transparent substrates 107a and 107b and a liquid crystal layer 107c interposed therebetween. Although not shown in the figure, other elements such as a driving circuit (including TFTs, signal lines, etc.) and alignment films are also provided between the transparent substrates 107a and 107b. On a side of the transparent substrate 107a facing the liquid crystal layer 107c, a black matrix 111 is provided for blocking light passing through the wiring region which does not contribute to the display. A light-transmitting area of each pixel is called a "pixel aperture". The ratio of the total area of all the pixel apertures with respect to the screen size is referred to as an aperture ratio.
The microlens array 105 is a group of microlenses 106 each having a size corresponding to three pixels of the LCD device 107. From the incident R, G and B beams (respectively collimated), the microlens array 105 forms focused spots of the three colors on respective pixels of the corresponding colors on a side (the lower side in FIG. 39) of the transparent substrate 107a on which the black matrix 111 is provided. Then, image signals are applied to control the respective pixels on which the focused spots are formed.
In a normal LCD device which is not provided with a microlens, light incident upon the black matrix 111 cannot contribute to the display, thereby lowering the light efficiency. On the other hand, in the above-described projection type color image display apparatus provided with the microlens array 105, light incident upon the microlenses 106 can be focused on the pixel apertures. Therefore, the amount of light which passes through the LCD device 107 is increased, thereby obtaining a brighter projection. If the size of a focused beam spot is smaller than the size of a pixel aperture, the light efficiency can be maximized. However, to realize such a condition, the following limitations exist.
The size of a focused beam spot after passing through a microlens is determined by the focal length f of the microlens and the degree of parallelization of the incident light (the spread angle of the light with respect to the principal ray). In the optical system illustrated in FIG. 39, the focal length f should be adjusted to be equal to the thickness of the transparent substrate 107a. However, in the LCD device production process currently employed in the art, a plurality of LCD devices are first produced on a large glass plate, and the large glass plate is then severed into pieces. Therefore, a very thin glass plate cannot be used as it may be warped during the production. The thickness d of a glass plate currently used in the LCD device production is about 0.7 to about 1.1 mm, and the refractive index n of the glass plate is about 1.52. Thus, the air-equivalent thickness (d/n) of the glass plate is about 460 .mu.m to about 730 .mu.m. Therefore, the focal length f of a microlens needs to be about 460 .mu.m or more. When the degree of parallelization of the illumination light is about .+-.3.degree., the focused spot size .PHI. is about 48 .mu.m (.PHI.=2.multidot.f.multidot.tan .theta. (.theta.:.+-.3.degree.)). Therefore, when the width of a pixel aperture is less than about 48 .mu.m, the focused beam spot spans beyond the aperture, thereby causing a focusing loss.
For example, in an LCD device employing a stripe arrangement and having a diagonal dimension of about 91 mm, an aspect ratio of about 3:4, and 480.times.[640.times.3(RGB)=1920] (vertical.times.lateral) pixels (a so-called "VGA (video graphics array)" type display), the lateral pitch of pixels is only about 38 .mu.m. Therefore, the focused beam spot is likely to span beyond a pixel aperture. It is even likely that an R beam spot overlaps the adjacent B and G pixels, thereby deteriorating the color reproducibility in the projected image. This undesirable phenomenon is called "color mixing". In recent years, LCD devices have been made smaller in size with higher resolutions, thereby reducing the size of a pixel. Without special measures, the light efficiency may further decrease, whereby the brightness of the projection cannot be ensured and the undesirable color mixing cannot be prevented.
As one possible solution to such a problem, commonly-assigned Japanese Laid-Open Publication No. 9-114023 discloses a method in which focused beam spots formed by a first microlens array are imaged while being enlarged onto pixel apertures of an LCD device by means of a second microlens array. In this method, the focused beam spots formed by the first microlens array exist outside of the LCD device. Therefore, the beam spots can be efficiently focused on the pixel apertures without reducing the thickness of the transparent substrate 107a.
In the above-described conventional single-plate projection type color image display apparatus employing dichroic mirrors and a microlens array, the light efficiency can be improved since an absorption-type color filter is not used, but there are problems associated therewith as follows.
First, the illumination distribution across the screen cannot be made sufficiently uniform. In order to obtain a high quality projection, it is desirable to make the illumination distribution across the screen uniform as well as to improve the brightness, the color reproducibility,the resolution, and the like. Generally, when a LCD device is illuminated directly by a focusing system which employs a parabolic mirror or an ellipsoidal mirror, the illumination peaks at the center of the screen and decreases toward the periphery of the screen.
The system disclosed in Japanese Laid-Open Publication No. 9-114023 has a first microlens array provided near the LCD device. Therefore, a group of focused beam spots formed by the first microlens array take over the non-uniform illumination distribution created by the preceding focusing optical system. Then, the group of focused beam spots are re-imaged so as to correspond only to the pixels of the LCD device in the vicinity of the respective beam spots. Thus, the non-uniformity in the illumination distribution remains in the projected image.
The non-uniformity in illumination distribution is a problem not only in the single-plate system but also in a three-plate projection type color image display apparatus. As a solution to the non-uniformity in illumination distribution occurring in a three-plate projection type color image display apparatus, it has been proposed in the art to employ an optical system obtained by combining two so-called "fly-eye" lens arrays together. Moreover, Japanese Laid-Open Publication No. 7-181392 discloses a system employing two fly-eye lens arrays and a microlens array so as to improve the illumination distribution while also improving the efficiency in focusing light onto a pixel aperture.
Japanese Laid-Open Publication No. 7-181392 describes that the illumination distribution can be improved by illuminating secondary white light source images formed by the first fly-eye lens array (a secondary light source image is generally the same as a focused spot formed by a microlens) across the screen of the LCD device by the second fly-eye lens array so that the light beams from the secondary white light source images are superimposed on one another. It also describes that the focusing efficiency can be improved by focusing the secondary light source images arranged in a pattern similar to the pixel arrangement onto the corresponding pixel apertures by means of microlenses. However, in the system described in Japanese Laid-Open Publication No. 7-181392, the secondary light source images are white, and therefore the system requires a color separating optical system and a color synthesizing optical system. Thus, the system has the above-described problems associated with the three-plate system. In order to apply the system to a single-plate system, a color filter is necessary, whereby there is some absorption loss due to the color filter.
Japanese Laid-Open Publication No. 8-313847 discloses a projection type color image display apparatus intended for use with the single-plate system, in which fly-eye lens arrays and a microlens array are used in combination. In the projection type color image display apparatus, white light is separated into R, G and B beams by a color separating optical system. The apparatus includes a pair of first and second fly-eye lens arrays for each of the R, G and B beams. The secondary light source images formed by the first fly-eye lens array are closely arranged together in a pattern corresponding to the shape of a pixel aperture, thereby forming a group of R, G and B light source images. The R, G and B light source images illuminate the image display device while being superimposed on one another by means of the second fly-eye lens arrays which are separately provided for the respective colors, thus improving the illumination distribution. Then, the microlens focuses a predetermined color onto a pixel aperture. While the system does not have absorption loss due to the color filter, it requires a fly-eye lens optical system for each color, thereby increasing the number of components to be provided and thus the size of the overall optical system.
The second problem is that efficiency of utilizing "polarized light", which is involved in the display principle of an LCD device, cannot be sufficiently enhanced. This problem is also common to the single-plate and three-plate systems. In an LCD device, only a portion of randomly-polarized illumination light (linearly-polarized light) is transmitted by a polarizing plate provided on the light input side of the LCD device, and the linearly-polarized light is modulated by the LCD device. Then, an unnecessary portion of the modulated light is further removed through another polarizing plate provided on the light output side of the LCD device, thereby displaying an image. More than half of the illumination light is removed and lost as the light passes through the first polarizing plate.
For improving the efficiency of utilizing the polarized light, a polarization conversion method has been proposed in the art in which a PBS (polarization beam splitter) is employed to align the polarization direction of the incident light before it is incident upon the LCD device. Japanese Laid-Open Publication No. 7-181392, supra, also discloses a polarization conversion method using a PBS. However, a light beam obtained as a result of the polarization direction alignment is parallel to a light beam before polarization separation, thereby doubling the cross-sectional area of a light beam, and thus increasing the size of the optical system. Moreover, when the display area of the LCD device is small, the effective amount of light focused on the display plane is reduced.
Moreover, Japanese Laid-Open Publication No. 8-304739 discloses a system where two fly-eye lens arrays and a PBS having a strip array pattern are used in combination so as to improve the illumination distribution uniformity and also improve the light efficiency by the polarization conversion in a small space. However, the secondary light source images formed by the system are white, and the system only contemplates the application to a three-plate projection type color image display apparatus.